ORA-30036: unable to extend segment by 8 in undo tablespace 'UNDOTBS'

You have enough undo tablespace but you can't use free area althoug it is free.
or
there is no active transaction in your database but undo tablespace is full?


--firstly select free space by this select

SQL> select sum(bytes) from dba_free_space where tablespace_name='UNDOTBS';

SUM(BYTES)
----------
1080098816

--select extent total size by grouping status

SQL> SELECT DISTINCT STATUS, SUM (BYTES), COUNT (*)
  FROM DBA_UNDO_EXTENTS
GROUP BY STATUS;

STATUS    SUM(BYTES)         COUNT(*)                               
--------- ------------------ ---------------- 
ACTIVE              18481152               17 
EXPIRED             14417920              220 
UNEXPIRED         2064187392             8695 

SQL> show parameter undo

NAME                                 TYPE                             VALUE
------------------------------------ -------------------------------- -----------
undo_management                      string                           AUTO
undo_retention                       integer                          3600
undo_tablespace                      string                           UNDOTBS

you can set "_undo_autotune" and undo_retention parameters  to trigger clearing unexpired extents which no one in use.

ALTER SYSTEM SET "_undo_autotune"=FALSE;
ALTER SYSTEM SET undo_retention=3600;